Use field value function to get html signature

The  function is used for all other tokens on the contact object and will better access
the right value and return an empty string rather NULL if not set.

I hit this as an enotice writing a test - but it is possibly the cause of this issue
eileenmcnaughton/nz.co.fuzion.civitoken#58 (comment)

hence targetting the rc

The tokens() function does weird things if you pass it NULL

CRM/Contact/Tokens.php

@@ -316,7 +316,7 @@ public function onEvaluate(TokenValueEvent $e) {
             ->tokens('contact', $token, "cs={$cs}");
         }
         elseif ($token === 'signature_html') {
-          $row->format('text/html')->tokens('contact', $token, html_entity_decode($row->context['contact'][$token]));
+          $row->format('text/html')->tokens('contact', $token, html_entity_decode($this->getFieldValue($row, $token)));
         }
         else {
           parent::evaluateToken($row, $this->entity, $token, $row->context['contact']);